Receptor Proteins
- Signal transduction is the series of steps that govern the appropriate response by the cell
- These are triggered by the binding of the ligand to its specific receptor
Transmembrane Receptor Proteins
- Receptors that are located in the cell membrane have an external binding site and an internal region which extends into the cytoplasm
- These are transmembrane proteins as they extend right across the width of the membrane
- Transmembrane receptors are characterized by:
- Hydrophilic amino acid regions at either end of the protein that are contact with the aqueous solution inside and outside the cell
- A hydrophobic amino acid region within the membrane that is in contact with the hydrophobic tails of the phospholipids inside the cell membrane
- Some ligands bind to these receptors instead of entering the cell cytoplasm
Transmembrane Receptor Diagram
The cell surface membrane has many different components, including transmembrane proteins, e.g. glycoproteins, that function as receptors
Intracellular receptors
- Nonpolar, hydrophobic ligands, eg. steroid hormones, can diffuse through the phospholipid bilayer
- These ligands bind to receptors in the cytoplasm or on the DNA in the nucleus of the cell
- Steroid hormones such as estradiol will bind to the receptor molecule and activate it so that protein synthesis is initiated
Intracellular Receptor Diagram
Estradiol is a steroid hormone that diffuses across the cell membrane and binds to an intracellular receptor in the cytoplasm of target cells